were to protect the wooden structure of the bridge. True covered bridges are defined by a timber truss (or frame) that distributes the weight of the load-bearing deck. The truss design determines how long a span it can have and how complex it is to build. This makes them different than simple timber beam bridges or other similar structures. Of course, to be a covered bridge, that timber truss must also be covered. But Why Build a Bridge Out of Wood? Low cost of building materials. Covered bridges date back to a time in the country when almost everything was made of wood because of its abundance. Working with wood, like any material, has its advantages and disadvantages. Timber is lighter than stone and cheaper than iron or steel, but still very strong. However, wood also deteriorates faster. Why Is It Necessary to Cover a Bridge? The fundamental structural purpose for covering a bridge is to shield the trusses and deck from the elements. Because wooden bridges with exposed superstructures are vulnerable to rot, covering and roofing the bridges protects the trusses from the weather, and so they last longer. Bridge engineers pointed out that uncovered wooden bridges have a 20-year lifespan, but if you covered a wooden bridge with a roof it could last upwards of 100 years.

Protect structural bits from the rain and sun. Wood bridges that are covered last 100+ years vs 20 years for uncovered.

Originally, costs, structural and esthetics. Bridges tend to experience torsion for a variety of factors. The covered part historically provides additional rigidity without supports below into the water. It also protects the covered area from bad weather.

As building materials and engineering improved its now largely about esthetics.

I'm glad you didn't say redpill me on wooden bridges.

Somewhere I heard that this was to keep the horses from getting scared when they saw the water. Any info on this?